1. Field of the Invention
This invention relates to an outboard motor, and in particular, to a lubrication system for an outboard motor.
2. Description of the Related Art
An example of a conventional outboard motor is shown in Japanese Publication No. 2003-065174. The outboard motor disclosed in this publication comprises an internal combustion engine, which serves as a drive source and constitutes the upper part of the motor. A propeller constitutes the lower end of the motor and is driven by the engine. A lubricant pan is provided for reserving lubricant (e.g. oil) for the engine. A lubricant feeding pump pressurizes the lubricant in the lubricant pan and feeds the lubricant to various parts of the engine. In this manner, the parts of the engine are lubricated. As is typical with outboard motors, the outboard motor of the publication pivots about a pivot point at its upper part on a boat body so that its lower end can be swung up in a rearward direction and down in forward direction.
Upon starting the engine, the lubricant replenishing pump is actuated to feed lubricant to the various parts of the engine. As mentioned above, in this manner, the various parts of the engine are lubricated to enable continuous operation of the internal combustion engine. With the driving force from the internal combustion engine, the propeller is rotated to propel the boat. During propulsion, the outboard motor is swung back and forth with respect to the pivot point to produce an intended angle relative to the plumb line while the boat is being propelled. In this manner, the propeller remains under the water surface and the altitude of the boat can be changed to attain an intended propulsion state. When the outboard motor is not in use (e.g., as when mooring), the main part of the outboard motor is typically swung up so that the propeller is positioned above the water. When the outboard motor is to be used again, the main part of the outboard motor is swung back down so that the propeller is replaced under the water surface.
It is generally advantageous that an outboard motor be small in size and light in weight. In a similar manner, it is generally advantageous that the lubricant pan of an outboard motor is also made as small as practicable. On the other hand, while the outboard motor is generally required to be of small size and light weight, it is typically operated at high speeds and therefore high output is generally required. Therefore, in many outboard motors, a large amount of lubricant is consumed per unit time. As a result, the lubricant pan must be frequently replenished. The frequent replenishing of the lubricant pan can be cumbersome. If the capacity margin of the lubricant pan is reduced to reduce the size of the outboard motor, the replenishing frequency of replenishing increases.
One solution to the above-mentioned shortcomings is shown in Japanese Patent Applications JP-A-Sho 59-215911 and JP-U-Hei 7-22006. According to these publications, the main part of the outboard motor includes a lubricant tank disposed separately from the lubricant pan for reserving lubricant. A lubricant replenishing pump is provided for transferring lubricant from the lubricant tank to the lubricant pan. The lubricant is automatically fed to the lubricant pan in response to a detection signal if the lubricant level falls below a lower limit while the internal combustion engine is in operation.
However, there are disadvantages with the above described outboard motors. For example, when the internal combustion engine starts operation, the lubricant in the lubricant pan is fed to various parts of the engine, which causes the level of the lubricant in the lubricant pan to be lowered while the engine is in operation. Moreover, during operation, the main part of the outboard motor and the boat that is being propelled are moved in response to the resistance of waves. In addition, the main part of the outboard motor changes its tilt angle at it is swung about its pivot axis. Therefore, the level of the lubricant in the lubricant pan fluctuates and cannot be maintained at a constant level.
In such a situation, the lubricant level in the lubricant pan is largely different from the level that would correspond to the lubricant amount in the whole engine. Therefore, if the lubricant pan is replenished according to the detected lubricant level, the replenishment amount might be too much or not enough.
Therefore, it would be desirable to address the above-noted shortcomings in the prior art and to provide a lubrication system in which a lubricant pan is replenished with the neither too much nor too little lubricant from a lubricant tank disposed separately from the lubricant pan. Another object of the present invention is to provide more effective lubrication for parts of the internal combustion engine to be lubricated.